A fuel assembly normally comprises a plurality of vertical fuel rods, which, in spaced relationship, are arranged between a pair of end parts referred to as a bottom tie plate and a top tie plate. The bottom tie plate is provided with continuous, vertical channels for conducting coolant through the bottom tie plate and into the spaces between the elongated elements in order to cool the elongated elements during a nuclear reaction.
For a correct mutual positioning of the elongated elements, spacers are arranged at several levels along the fuel assembly.
The lower ends of the elongated elements, which ends are arranged in the assembly, are normally provided with plugs which are arranged resting on and guided in openings, so-called plug holes, in the bottom tie plate. The upper ends of the elongated elements, which ends are arranged in the assembly, are arranged free or guided in the top tie plate.
To be able to handle a bundle of elongated elements as one unit, one or more elongated elements are arranged to keep the bundle together. These elongated supporting elements may, for example, consist of ordinary fuel rods in a boiling reactor and are then referred to as supporting fuel rods, or of control rod guide tubes in a pressurized-water reactor. These supporting elements are normally locked in the bottom tie plate. Non-supporting elongated elements are prevented from lifting from the bottom tie plate, due to lifting forces caused by the flow of the coolant, partly by their own weight, partly by friction in the spacers and, in addition, by a mechanical stop means consisting of the top tie plate.
Other elongated elements, which may be locked in the bottom tie plate, are so-called part-length rods and spacer holder rods. The task of a spacer holder rod is to limit the movement of the spacers in the axial direction because of the coolant flowing upwards through the fuel assembly. In a fuel assembly for a boiling water reactor, the spacer holder rods may consist of one or more ordinary fuel rods or of water-conducting rods. In a fuel assembly for a pressurized-water reactor, it is normally the control rod guide tubes which have the function of holding the spacers.
Usually, the bottom tie plate is made of stainless steel to have good mechanical properties. The top tie plate may be constructed in a plurality of different ways depending on the function it has in the relevant fuel assembly. At least some of the elongated elements is usually provided with an end plug which is inserted into or through the bottom tie plate for locking the elongated element to the bottom tie plate. The end plugs are normally made of a zirconium-base alloy. It is desirable for the end plugs to be detachably arranged in relation to the bottom tie plate so that they can be mounted and dismantled, respectively, for replacing failing parts.
Especially in fuel assemblies comprising part-length fuel rods, which do not reach from the bottom tie plate and up to the top tie plate, it is important to ensure that the part-length fuel rods are not lifted out and lose their guidance in the bottom tie plate because of the friction from the coolant flowing upwards through the bottom tie plate. Part-length rods, which extend from the bottom tie plate up towards the top tie plate without reaching it, are guided above the bottom tie plate, by the mentioned spacers, at different levels along the bundle. In the event that the guiding in the bottom tie plate is lost by a fuel rod being displaced upwards in the fuel assembly, long free ends may be put into vibration and cause abrasion on adjacently located fuel rods and, in the worst case, cause penetration of one or more fuel rods such that fissionable material may enter into the coolant.
Supporting rods are usually locked to the bottom tie plate by a threaded joint. It is, for example, common for the supporting rod to be provided with a threaded end plug which is passed through a continuous plug hole arranged in the bottom tie plate and is fixed to the bottom tie plate by arranging a nut on the lower side of the bottom tie plate, when arranged in the reactor. Another common method is to provide the plug hole in the bottom tie plate with a thread for cooperation with the threaded end plug. A difficulty in connection with threaded joints is that the threaded joint, because of corrosion, may seize after operation of the reactor. Also particles which are scaled and brought with the coolant may become deposited on the threaded surfaces and give rise to seizing. Corrosion is particularly difficult when the threaded plug consists of a zirconium alloy and the thread in its fixing element, for example the bottom tie plate or a nut, is made of stainless steel.
When designing a fuel assembly of the above-mentioned type, it is desirable to be able to handle, as far as possible, the parts included in the fuel assembly from the upper end of the fuel assembly. It is not possible to handle the parts included from above when using a threaded joint where the rod is to be fixed with a nut to the underside of the bottom tie plate. Another difficulty with this type of threaded joint is that the nut and the plug project from under the bottom tie plate in an area where the coolant flow is very turbulent. The turbulent flowing coolant may give rise to vibrations and hence abrasion of the end plug. This implies that, when designing the fuel assembly, it is also desirable to avoid parts which project from under the bottom tie plate.
It is possible to handle the parts included from above when using a threaded joint of the type where the bottom tie plate comprises threaded holes into which the threaded end plugs of the rods are screwed. One difficulty is during dismantling when the rod is turned in the opposite direction. On the one hand, it is awkward to rotate a rod which has a length of four metres and, on the other hand, the threaded joint may seize. In the event that the thread seizes when dismantling it, there is a risk that the rod is subjected to turning forces and, in the worst case, that it is twisted off. In addition, it involves considerable handling problems since it is difficult to reach and mount and dismantle rods with a shorter length than adjacently located rods, and especially if the thread seizes.
Locking normal, that is, non-supporting, fuel rods to the bottom tie plate in a corresponding way is unnecessary and disadvantageous. There is no need of attaching normal fuel rods in this way since the forces from the upwardly flowing flow are much lower than the forces acting on the bundle when handling, such as lifting, the entire bundle.
The object of the invention is to achieve a fuel assembly with a bottom tie plate which permits a detachable locking of the end plugs of elongated elements, both supporting and non-supporting and full-length and part-length elements, to the bottom tie plate.